LEPTON. 101 



edges of which can be either closed or expanded, as in the 

 foot of the Nucula. As may be imagined from the size of 

 the foot, it has the power of crawling about very freely, 

 and sometimes it also swims inverted on the surface of the 

 water in the manner of the gasteropods, the hinder part of 

 the foot being then unfolded into a disc : but its favourite 

 position is that of repose, suspended freely in a perpendicu- 

 lar position, with the umbones downwards, by three or four 

 threads, so fine, that they cannot be seen by the naked eye, 

 and even with a magnifier can only be observed in certain 

 positions of light. The byssal aperture appears to be about 

 the centre of the foot. When the animal is withdrawn, 

 the shell can be completely closed, but usually the valves 

 are held a little open, and the mantles protruded beyond 

 them. The siphon is at the longer end of the shell. The 

 branchial leaflets are two on each side, attached to the 

 inside of the mantle." 



As long ago as June, 1835, this remarkable creature 

 had been drawn and noted by Mr. Clark, at Exmouth, but 

 his notes remained unpublished. They agree in most par- 

 ticulars with those of Mr. Alder, the only essential differ- 

 ence being his observation " of two very small orifices, a 

 little separate from each other, above the posterior end of 

 the foot.'" 



This scarce shell is very rarely obtained entire, dead 

 single valves being the usual condition of cabinet specimens, 

 and good examples of them are not common. It inhabits 

 the laminarian and coralline regions chiefly, towards the 

 south and west. Montagu, its discoverer, took a solitary 

 example in Salcomb bay. On the south coast of England it 

 occurs, — in fifteen fathoms, West Bay of Portland, and in 

 twenty-eight fathoms off Plymouth (M'Andrcw and E. F.); 

 Exmouth (Clark) ; Torbay (Alder) ; most numerous at 



